They found that these companies prefer more privileged candidates by focusing their talent search at top-flight universities like Oxford and Cambridge, whose students tend to come from richer backgrounds.

High school history also made a difference. Up to 70% of job offers at these leading firms went to applicants who attended private schools, or schools that select their pupils based on academic performance. Those schools make up just 4% and 7% of the wider population.

Trainees at some top law firms are more than five times more likely to have attended a private school than the population as a whole.

"Elite firms seem to require applicants to pass a 'poshness test' to gain entry," said Alan Milburn, chair of the Social Mobility and Child Poverty Commission that conducted the research.

Young job applicants with strong academic grades and abilities, but without money in the bank, were being excluded, he said. They were being put at a disadvantage because of differences in social background, experiences and accents.

"If you're getting picked at nineteen and you're from a socially less desirable background, you haven't had three years to learn to speak properly and iron out some of the embarrassments that would be an impediment to you," said a recruiting manager at one company.

Another mentioned the kind of holidays or languages an applicant speaks can help build a rapport with an interviewer.

Plus candidates from a similar background make working easier, with one respondent saying,"I can write...an obscure comment in the margin and you'll know exactly what I'm talking about. You get my jokes...we get each other and that's hugely efficient."

The authors said firms' are denying themselves talent, and recommended that companies use strategies to ensure more diverse student recruitment, along with greater support for those applicants to navigate selection processes.